In many radio frequency, RF, applications power amplifiers are employed to amplify high frequency signals. Because the RF amplifiers are biased to provide substantially high output power, they exhibit nonlinear responses to some degree. Consequently, in response to an increase in the output signal power, such RF amplifiers generate intermodulation IM components, which may have frequencies that are outside a desired frequency band.
One solution to eliminate the consequences of the nonlinear response of the amplifier is to employ multiple amplifiers each configured to amplify a predetermined carrier signal. For example, in a mobile communications environment, the base station sends multiple carrier signals in accordance with time division multiple access (TDMA) scheme, or in accordance with code division multiple access (CDMA) scheme. Each carrier frequency in TDMA scheme corresponds to one of the users in a specifiable cell. Furthermore, each pseudocode in CDMA scheme corresponds to one of the users in a specifiable cell. Because the base station has to communicate with many users in the corresponding cell, the intermodulation IM components increase with the number of the users. Thus, the use of a separate amplifier for each carrier signal substantially eliminates the generation of intermodulation IM components. However, this approach is costly and may not be commercially feasible in many applications.
Another approach is to employ an analog linearizer, such as 10 as illustrated in FIG. 1. For purposes of illustrating the operation of linearizer 10, it is assumed that a two-tone signal is provided to the linearizer. Basically, a radio frequency signal represented by frequency components 22 is fed to a power amplifier 12. Amplifier 12 generates additional intermodulation IM frequency components 24 because of its nonlinear response characteristics. Signal components 22' correspond to an amplified version of signal components 22. The function of linearizer 10 is to substantially eliminate frequency components 24, as explained in more detail below.
Linearizer 10 includes a signal cancellation circuit 26 coupled to an error cancellation circuit 28. Signal cancellation circuit 26 has an upper branch that includes power amplifier 12, and a lower branch that provides the input signal of the linearizer to an input port of an adder 16 via a delay element 15. The other input port of adder 16 is configured to receive the output signal generated by power amplifier 12, via an attenuator 14. As a result, the output port of adder 16 provides signal components 24', which correspond to the attenuated version of intermodulation IM frequency components 24. The purpose of delay element 15 is to assure that the input signal provided to adder 16 through the lower branch is aligned with the input signal provided through the upper branch.
Error cancellation circuit 28 also includes an upper branch that is configured to provide the output signal generated by amplifier 12 to an adder 20 via a delay element 17. The lower branch of error cancellation circuit 28 includes an amplifier 18, which is configured to receive the attenuated intermodulation components 24'. Amplifier 18 generates an amplified version of signal 24' which is substantially equal to intermodulation component 24. As a result, the output port of adder 20 provides signal components 22' without the distortion caused by amplifier. The purpose of delay element 17 is to assure that the signal provided through the lower branch is aligned with the direct signal provided in the upper branch.
The feedforward linearizer described in FIG. 1 has some disadvantages. For example, it is not able to adapt to signal changes. Furthermore, for wide-band input signals in the microwave frequency range, adjusting the delay in delay elements 15 and 17 is difficult. A small delay misalignment may lead to serious signal distortion. In order to provide a delay alignment between the upper and lower branches of the two cancellation circuits, some linearizers have been suggested that attempt to align the signal by trial and error during the operation. These linearizers employ a delay adjuster to achieve the intended delay alignment. However, the trial and error approach provides only limited accuracy and may lead to unacceptable output signal response.
For signals or the microwave frequency range, the bandwidth accommodated by power amplifier 12 is relatively small. Amplifiers that accommodate a large bandwidth are expensive. Thus, equalization for the power amplifier is required to increase the operating bandwidth so that the frequency response of the power amplifier is substantially flat. The prior art feedforward linearizers direct all the linear distortion caused by delay misalignment and the non-linear distortions caused by of the power amplifier to the auxiliary amplifier in the error cancellation loop. The auxiliary amplifier is designed as a class A amplifier. The distortion generated by the auxiliary amplifier itself is not recoverable. Thus, a high-accuracy class A amplifier that handles high power input is required in the error cancellation loop, which is expensive and difficult to design.
Thus, there is a need for a feedforward linearizer that employs an effective digital signal processing technique that provides delay alignment and equalization to suppress intermodulation components, by an arrangement that is both effective and economical.